The current disclosure relates to electronics, and more specifically but not exclusively, to heat dissipation for active semiconductor devices.
Wireless communication systems may include base stations that transmit electromagnetic (EM) signals. The base station transmitters employ radio-frequency (RF) power amplifiers to amplify RF EM signals for transmission. RF power amplifiers are active semiconductor-based devices that may employ any of various different technologies to achieve amplification, e.g., LDMOS (laterally diffused metal oxide semiconductor), GaN, or GaAs. Typically, a base-station transmitter comprises a series of amplification stages—which may be referred to as a lineup—that increases in power level from lower-level amplification to higher-level amplification. A final-stage RF power amplifier typically generates more heat than an earlier-stage RF power amplifier. A lineup of RF power amplifiers comprising final-stage and/or pre-final-stage power amplifiers may be mounted onto a printed circuit board (PCB) that comprises additional electronic components.
FIGS. 1A-1D show various views of exemplary conventional packaged RF power amplifier device 100. FIG. 1A is a first orthogonal view of amplifier device 100. FIG. 1B is a second orthogonal view of amplifier device 100 of FIG. 1A. FIG. 1C is a third orthogonal view of amplifier device 100 of FIG. 1A. FIG. 1D is a perspective view of amplifier device 100 of FIG. 1A. Amplifier device 100 is packaged in an earless flanged LDMOS package.
Specifically, amplifier device 100 comprises drain lead 101, gate lead 102, source lead 103, and encapsulant 104. Drain lead 101 and gate lead 102 are in the form of fins. Source lead 103 forms the earless flange of amplifier device 100 and may be referred to as flange 103. Note that LDMOS packages with eared flanges (not shown) have flanges that extend further out to the sides, where the extensions may have slots or holes for screws or similar attachment means. Also note that LDMOS packages with earless flanges are sometimes referred to elsewhere as flangeless packages. Encapsulant 104 may comprise, for example, ceramic and/or epoxy. Amplifier device 100 also comprises a semiconductor die that is encapsulated by encapsulant 104 and not visible in FIGS. 1A-1D.
The semiconductor die comprises a power transistor whose terminals are conductively connected to the corresponding external leads. In other words, the power transistor's drain, source, and gate terminals are conductively connected to drain lead 101, source lead 103, and gate lead 102, respectively. The transistor may also have a bulk-semiconductor terminal that is conductively connected to source lead 103. Note that the transistor may be a compound transistor where a plurality of smaller individual transistors are connected together so as to function like a single larger transistor. The leads 101, 102, and 103 are metallic—e.g., copper. Most of the heat generated by amplifier device 100 is dissipated through flange 103, which has relatively large surface area.
RF power amplifiers tend to generate a considerable amount of heat, where a higher power level generally correlates with more heat generated. Heat generated by RF power amplifiers needs to be dissipated to prevent device failure and in order to extend the operational life of the RF power amplifiers and/or nearby components. Conventional means of heat dissipation include the attachment of a finned heat sink to the RF power amplifier.
FIG. 2 is a simplified exploded perspective view of exemplary conventional RF power amplifier system 200. System 200 includes PCB 201, metal pallet 202, and two RF power amplifier devices 203, which each may be substantially similar to amplifier device 100 of FIGS. 1A-1D. PCB 201 has two apertures 204 for the two corresponding amplifier devices 203 and holes 205 for corresponding screws (not shown). Metal pallet 202 is a bulk metal plate comprising, for example, copper or aluminum. Pallet 202 has two depressions 206 for the bottom surfaces of the flanges of the two amplifier devices 203 and holes 207 for the above-mentioned corresponding screws.
Amplifier devices 203 are mounted onto PCB 201, where the drain, gate, and source leads of the amplifier devices 203 are electrically connected to corresponding contacts (not shown) on PCB 201. The flanges of the amplifier devices 203, which correspond to the source leads, are inserted through corresponding apertures 204 in the PCB 201 and into corresponding depressions 206 on pallet 202. PCB 201 also has mounted thereon additional components 208. Components 208 and amplifier devices 203 are electrically interconnected via traces (not shown) on PCB 201.
System 200 may further include a heat sink (not shown) whose top attaches to the bottom of pallet 202. The heat sink comprises, on the top, a bulk metal plate and, on the bottom, an array of metallic fins extending out from the bulk metal plate. The top of the heat sink may include screw holes for the above-mentioned corresponding screws for attachment to pallet 202 and PCB 201. A solid thermal medium or thermal grease (not shown) may also be applied between the pallet 202 and the heat sink to help facilitate proper thermal transfer between the mating surfaces.
Metals are relatively efficient heat conductors and a conventional heat sink conducts heat from the heat-generating device to the medium surrounding the fins—which is typically air—thereby effectively increasing the heat-dissipating surface area of the heat-generating device. However, the heat-dissipating capabilities of the pallet and heat sink combination are limited and, as result, the amplifier devices 100 may need to be spaced relatively far apart from each other so that they are not damaged by excessive heat from neighboring amplifier devices 100. More-efficient means of dissipating heat from an active device would lower the device's temperature, help extend its life, and provide additional benefits.